searcher: partially migrate to bstr

This commit causes grep-searcher to use byte strings internally for its
line buffer support. We manage to remove a use of `unsafe` by doing this
(by pushing it down into `bstr`).

We stop short of using byte strings everywhere else because we rely
heavily on the `impl ops::Index<[u8]> for grep_matcher::Match` impl,
which isn't available for byte strings. (It is premature to make bstr a
public dep of a core crate like grep-matcher, but maybe some day.)

grep-searcher/src/testutil.rs

@@ -1,10 +1,10 @@
 use std::io::{self, Write};
 use std::str;
 
+use bstr::B;
 use grep_matcher::{
     LineMatchKind, LineTerminator, Match, Matcher, NoCaptures, NoError,
 };
-use memchr::memchr;
 use regex::bytes::{Regex, RegexBuilder};
 
 use searcher::{BinaryDetection, Searcher, SearcherBuilder};
@@ -94,7 +94,8 @@ impl Matcher for RegexMatcher {
             }
             // Make it interesting and return the last byte in the current
             // line.
-            let i = memchr(self.line_term.unwrap().as_byte(), haystack)
+            let i = B(haystack)
+                .find_byte(self.line_term.unwrap().as_byte())
                 .map(|i| i)
                 .unwrap_or(haystack.len() - 1);
             Ok(Some(LineMatchKind::Candidate(i)))